This proposal sets forth an integrated set of structural and chemical studies to elucidate biologically relevant structure- reactivity correlations between the multiple polymorphic forms (isoforms) of the metal binding protein, metallothionein (MT). This protein is a normal constituent of mammalian tissues and it or related structures are found throughout nature. It is the principal intracellular binding site for the environmentally important toxic heavy metal, cadmium, and is thought to provide the means for its detoxification. A central role is also certain for MT in the metabolism of the essential metals Zn and Cu. Depending on the tissue and physiological status of the organism in which it is found, there are multiple isoforms of MT expressed and these most often contain more than one type of metal. The overall goal of the proposed research is to rationalize on the basis of detailed solution structural studies any differences among the isoforms in their reactivity in ligand exchange and interprotein metal exchange reactions thought to be undergone by the protein in vivo. The structural probes to be used are 113Cd and 2H NMR and they will be employed interactively to provide complementary information regarding the environments of the metals at each of the sites in the two cluster domains and the protein structure surrounding these sites. The following specific goals will be pursued: 1) to document structural differences between the multiple MT isoforms from rabbit and sheep liver which appear to alter their modes of interaction with Cd and Zn, 2) to characterize any differences ln reactivity between the isoforms in ligand exchange and interprotein metal exchange reactions, 3) to characterize the mixed-metal products resulting from Cu(I) displacement of Cd and Zn from MT and direct metal exchange between CuMT and CdMT and ZnMT, and 4) to assess the biological relevance of the reactions in 3) by comparing their products to those of native rabbit liver Cu, ZnMT.